problem with inheritance

Question

class OGraph {
public:
    OGraph() { }
    virtual ~OGraph();

    virtual bool is_directed()=0;
};

class OUGraph : public OGraph {
public:
    OUGraph() {  }
    bool is_directed() { return false; }
    ~OUGraph() {}
};

but when I do this

OUGraph myGraph();
cout << myGraph.is_directed();

I get the following error:

Undefined symbols: "typeinfo for OGraph", referenced from: typeinfo for OUGraphin main.o "vtable for OGraph", referenced from: OGraph::OGraph()in main.o mkdir -p dist/Debug/GNU-MacOSX "OGraph::~OGraph()", referenced from: g++ -o dist/Debug/GNU-MacOSX/opengraph build/Debug/GNU-MacOSX/OGraph.o build/Debug/GNU-MacOSX/main.o
OUGraph::~OUGraph()in main.o OUGraph::~OUGraph()in main.o ld: symbol(s) not found collect2: ld returned 1 exit status make[2]: * [dist/Debug/GNU-MacOSX/opengraph] Error 1 make[1]: [.build-conf] Error 2 make: ** [.build-impl] Error 2

BUILD FAILED (exit value 2, total time: 568ms)

Jim Buck · Accepted Answer · 2011-01-09 02:57:00Z

4

This:

OUGraph myGraph();

is declaring a function called myGraph that returns a OUGraph. You need to instead have:

OUGraph myGraph;

This declares a variable, as you intend.

answered Jan 9, 2011 at 2:57

Jim Buck

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1 Comment

zzzbbx Over a year ago

I am sorry, I pasted the wrong code. I just updated it, with the error message I get.

Edward Strange · Accepted Answer · 2011-01-09 03:16:15Z

2

It's the same old thing all new C++ people fall for.

OUGraph myGraph();

You've not created a variable called myGraph, you've declared a function called myGraph that returns an OUGraph. Remove the "()".

You also apparently never defined your base's destructor anywhere. Even if it's empty you'll need to define it since you declared it in your class definition.

edited Jan 9, 2011 at 3:16

answered Jan 9, 2011 at 2:58

Edward Strange

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Comments

fuzzyTew · Accepted Answer · 2011-01-09 03:14:25Z

1

You have no implementation provided for virtual ~OGraph();. Either provide an implementation, specify it as pure virtual with = 0;, or just get rid of it.

The compiler is expecting to attach type info for the class to the implementation of its first virtual method. Your first virtual method is undefined, so the implementation is never generated, and you get related linking errors.

Additionally, as everyone is saying, you should remove the parentheses from your definition of myGraph.

answered Jan 9, 2011 at 3:14

fuzzyTew

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Comments

chustar · Accepted Answer · 2011-01-09 02:58:41Z

0

If you're constructing a new object based using a 'default' constructor, I believe you call it without the empty parenthesis.

So, you would use OUGraph myGraph not OUGraph myGraph()

answered Jan 9, 2011 at 2:58

chustar

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